nvme-pci: flush initial scan_work for async probe

The nvme device may have a namespace with the root partition, so make
sure we've completed scanning before returning from the async probe.

Fixes: eac3ef26 ("nvme-pci: split the initial probe from the rest path")
Reported-by: Klaus Jensen <its@irrelevant.dk>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Klaus Jensen <k.jensen@samsung.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>